THE BREATHING SYSTEM 



483 



of the individual. Athletes usually have 

 slightly more vital capacity than do non- 

 athletes, women slightly less than men. 



Some air, about 1000 cc, is always left in 

 the lungs even after forced expiration. This 

 residual air is present to some extent in 

 lungs that have been removed from the 

 body. If the lungs of an unborn child are 

 placed in water they do not float, whereas 

 those of the newborn who has taken a 



for some time, with considerable distress, 

 but eventually he will be forced to breathe 

 again in a normal fashion. The rate of 

 breathing is altered by a number of condi- 

 tions arising from within as well as from 

 without the organism. Such conditions in- 

 clude emotional as well as physical strain 

 (Fig. 18-6). 



Breathing is a result of the coordinated 

 action of a great many muscles which re- 



+rocb«a 



diaphragm 



Fig. 18-7. The rote of breathing is controlled by the breathing center which is sensitive 

 to carbon dioxide in the blood. During activity the increased carbon dioxide content 

 of the blood stimulates the breathing center to bring about increased rate of breath- 

 ing. The opposite effect prevails when at rest. 



breath do. This test sometimes has legal 

 significance in suspected cases of infanti- 

 cide. 



CONTROL OF BREATHING 



Because the demands for oxygen vary 

 with activity, the rate and depth of breath- 

 ing vary to meet these demands. It is pos- 

 sible to change the rate of breathing volun- 

 tarily, although one does not need to think 

 in order to breathe. One can hold his breath 



quire a rather precise controlling mecha- 

 nism called the respiratory or breathing cen- 

 ter. This consists of a group of specialized 

 cells located in the medulla (Fig. 18-7). 

 Bursts of nervous impulses leave this cen- 

 ter over the phrenic nerves to the dia- 

 phragm and over the cervical sympathetics 

 to the rib muscles, causing both to contract, 

 resulting in inspiration. The rate at which 

 these contractions occur depends on several 

 factors influencing the respiratory center. 

 If the carbon dioxide content of the blood 



